netdev - Re: [PATCH net-next v6 04/12] net: homa: create homa_pool.h and homa

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Message-ID: <c5a5f0da-f941-4818-8dd7-b181cbfdca30@lunn.ch>
Date: Sat, 25 Jan 2025 01:46:02 +0100
From: Andrew Lunn <andrew@...n.ch>
To: John Ousterhout <ouster@...stanford.edu>
Cc: Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
	edumazet@...gle.com, horms@...nel.org, kuba@...nel.org
Subject: Re: [PATCH net-next v6 04/12] net: homa: create homa_pool.h and
 homa_pool.c

> > > +     homa_sock_lock(pool->hsk, "homa_pool_allocate");
> >
> > There is some chicken-egg issue, with homa_sock_lock() being defined
> > only later in the series, but it looks like the string argument is never
> > used.
> 
> Right: in normal usage this argument is ignored. It exists because
> there are occasionally deadlocks involving socket locks; when that
> happens I temporarily add code to homa_sock_lock that uses this
> argument to help track them down. I'd prefer to keep it, even though
> it isn't normally used, because otherwise when a new deadlock arises
> I'd have to modify every call to homa_sock_lock in order to add the
> information back in again. I added a few more words to the comment for
> homa_sock_lock to make this more clear.

CONFIG_PROVE_LOCKING is pretty good at finding deadlocks, before they
happen. With practice you can turn the stack traces back to lines of
code, to know where each lock was taken. This is why no other part of
Linux has this sort of annotate with a string indicating where a lock
was taken.

You really should have CONFIG_PROVE_LOCKING enabled when doing
development and functional testing. Then turn it off for performance
testing.

	Andrew